Field of the Invention
The invention relates to a method for creating an optical transmit signal, namely, an optical discrete multi-tone transmit signal. The invention further relates to a transmitter device as well as to a transmitter and receiver device.
Description of Related Art
Transmission of signals at high data rates (e.g. 100 Gb/s) per wavelength using intensity modulation and direct detection and discrete multi-tones (DMT) has become popular recently (see e.g. T. Chan et al., “400-Gb/s Transmission Over 10-km SSMF Using Discrete Multitone and 1.3-um EML's”, IEEE Photonics Technology Letters, Vol. 26, No. 16, Aug. 15, 2014). Some module vendors start to develop transponders for optical wavelength division multiplex (WDM) systems with single-channel transmission of 100 Gb/s using this technology. However, the optical signal-to-noise ratio (OSNR) requirements for these transponders are typically in the order of 40 dB, allowing only short transmission distances over standard single-mode fibers (SSMF) or requiring high effort on the transmission line, e.g. Raman amplification, which might not be possible in some network configurations. Furthermore, chromatic fiber dispersion needs to be compensated in typical known transmission configurations.
For example, an OSNR of 40 dB at the receiver after an 80 km transmission line of standard single-mode fiber, which leads to a loss of approximately 21 to 26 dB, can only be achieved with very high transmission power of 8 to 13 dBm. Distributed Raman amplification could reduce this power by approx. 5 dB, but would still require high transmission power values.
It is known that using electrical and optical components with a wide bandwidth will result in a reduction of the required OSNR, but will also increase the component cost. Furthermore, the optical bandwidth of an optical channel signal, which is to be transmitted over an optical WDM transmission link, should be constrained to an optical bandwidth lower than the channel spacing (i.e. the grid). For example, if the WDM system uses a grid of 50 GHz, the optical channel signals must be constrained to approximately 40 GHz. Additionally, when transmitting intensity modulated double sideband (DSB) optical signals over standard single-mode fibers (SSMF) chromatic dispersion induces power fading due to a phase shift of two corresponding sideband components depending on the dispersion parameter, the fiber length and the frequencies of the optical carrier signal and the modulation signal. That is, the RF (radio frequency) power of the modulation components of the optical DSB signal, which is detected at the far end of the optical fiber (or more generally, at the far end of the optical transmission path), shows drop-outs as a function of the modulation frequency. Of course, this effect also leads to a corresponding dependency of the signal-to-noise ratio of the RF signal received. In practice, of course, the frequency responses of all components at the transmitter and receiver side additionally influence the frequency-dependency of the power of the RF signal received and thus the signal-to-noise ratio of the signal received. If an optical DMT signal is used in an optical transmission system, the signal-to-noise ratio of the sub-channel signals is thus dependent on the chromatic dispersion and the frequency responses of the transmitter and receiver devices.
It is therefore an object of the present invention to provide a method to improve the transmission of an optical DMT transmit signal over an optical transmission line, especially to increase the OSNR tolerance, without costly modifications within the optical link or costly modifications of the optical transmitter device or the optical receiver device. It is a further object to provide an appropriate optical transmitter device and an appropriate optical transmitter and receiver device.